The present invention relates to solid solution nanoparticles of host materials doped with one or more rare earth elements. The present invention additionally relates to solution methods for preparing from solution nanoparticles of rare earth element doped host materials. The present invention also relates to luminescent devices incorporating nanoparticles of rare earth element doped host materials.
Halide salts have received world-wide attention as materials for a myriad of photonic applications. This results from a chemistry in which the ionic species are of generally greater atomic mass and weaker bonding than oxide-based compounds. This intrinsically results in a greatly enhanced theoretical transparency-hence there is substantial interest from telecommunication companies looking for ultra-low loss halide (predominantly fluoride) optical fibers for long-haul communications. When halide materials are doped with luminescent ions (e.g., the rare-earths), the weak bonding between relatively heavy atoms further results in a reduced influence of the host on the dopant, thereby causing radiative emissions not available from the equivalent ion in, for example, oxide-based systems. Accordingly, the halides are said to be of low-phonon energy and thereby enabling of a wealth of applications. Pertinent examples are optical amplifiers at the 1.3 xcexcm telecommunications window, upconversion light sources providing virtually any emission across the near-ultraviolet, visible, and near-infrared spectrum, color display materials (flat panel phosphors and volumetric monoliths), and long-wavelength sources for infrared imaging, atmospheric sensing, and military counter-measures. Collectively, these few applications represent a multi-trillion-dollar-per-year commerce.
Unfortunately, in most cases, conventional processing methods have failed in their efforts to produce optical components such as fibers with the promised near-intrinsic material properties much less expensively. Resultantly, rare-earth doped halide amplifiers are sold on a very small scale by a very small number of companies. Only applications utilizing relatively small-scale consumption of halide materials currently are sought-generally based on the halides"" low-phonon energy nature and resultant luminescent properties.
In particular, conventional processing methods have failed to produce significant concentrations of rare earth element ions in metal halide salts. Jones et al. J. Crystal Growth, 2, 361-368 (1968) discloses that the concentrations of rare earth ions in LaF3 crystals grown from a melt is limited to levels ranging from 25 mole percent for samarium (Sm) to less than 1 mole percent for ytterbium (Yb). Only cerium (Ce), praseodymium (Pr) and neodymium (Nd) are disclosed as being completely soluble in LaF3.
Kudryavtseva et al., Sov. Phys. Crystallogr., 18(4), 531 (1974) disclosed that higher solubilities can be obtained when melt-grown crystals are quenched into water. The disclosed improved solubilities in LaF3 range from 65 mole percent for Sm down to 5 mole percent for lutetium (Lu).
Neither prior art publication discloses the direct preparation of rare earth element doped metal halide salt nanoparticles. A need exists for a method by which such particles may be directly prepared, as well as for materials having increased levels of rare earth element dopants from terbium (Tb) to Lu.
The present invention addresses these needs. It has now been discovered that solid solution rare earth element doped nanoparticles may be prepared by solution processing techniques, particularly in connection with either reactive atmosphere methods and solution synthesis methods at temperatures significantly below the melt temperatures of the materials.
The synthesis methods of the present invention may be employed to prepare nanoparticles doped with Tb, dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), Yb and Lu at levels heretofore unknown in the art. Therefore, according to another aspect of the present invention, a rare earth element doped composition is provided doped with up to about 60 mole percent of one or more rare earth elements selected from Tb, Dy, Ho, Er, Tm, Yb and Lu wherein the composition is optically transparent to wavelengths at which m excitation, fluorescence, or luminescence of the rare earth elements occur, and the level of rare earth element is greater than about 50 mole percent for Tb and Dy, greater than about 40 mole percent for Ho, greater than about 30 mole percent for Er, greater than about 20 mole percent for Tm, greater than about 10 mole percent for Yb and greater than about 5 mole percent for Lu. The composition may also be doped with a rare earth element other than Tb, Dy, Ho, Er, Tm, Yb and Lu in amounts providing a total rare earth element content of 90 mole percent or greater.
Preferred compositions include halides and chalcogenides of lanthanum (La), lead (Pb), and the Group II metals of the Periodic Chart, e.g. beryllium (Be), magnesium (Mg), Calcium (Ca), strontium (Sr) and barium (Ba). Semiconductor elements such as arsenic (As), and compounds of Group IIIA and IVA of the Periodic Chart may also be used, including, but not limited to silicon (Si), gallium arsenide (GaAs), gallium nitride (GaN), indium nitride (InN), and the like.
Thus, when a lanthanum halide is employed, compositions in accordance with the present invention will have the stoichiometric formula MyLa1-yX3, wherein M is a rare earth element selected from Tb, Dy, Ho, Er, Tm, Yb and Lu; X is a halogen; and y is selected to provide a rare earth element dopant content within the above-described molar percent ranges for the rare earth element of M. The synthesis method of the present invention overcomes the stability problems encountered when synthesizing rare earth element doped halide salts of hygroscopic metals. This makes possible the preparation of rare earth element doped halide salts of these metals.
Two distinct methodologies may be employed to prepare the rare earth element doped metal halide salts of the present invention. In one method, the synthesis is performed by a reactive atmosphere treatment of multicomponent metal hydrous oxides, for example, by reaction with a halogenating gas. The present invention thus also provides a method of making nanoparticles by providing a substantially homogeneous multicomponent starting material containing a halide-forming metal and halide-forming rare earth element compound, and heating the starting material with an excess of a hydrogen halide or a halogenating gas in an atmosphere substantially free of water vapor at a temperature at which halogenation will occur, the temperature preferably being below the melt temperature of the lowest melting point component of the mixture.
The starting material may be a homogeneous physical mixture of compounds of the individual components (e.g., a mixture of nanosized particles) or a singular compound combining all of the components on an atomic scale of uniformity. Halide forming metal and rare earth element compounds include oxides, hydrous oxides and hydroxides.
According to another embodiment of this aspect of the invention, the rare earth element doped metal halide salts of the present invention may be prepared from an aqueous solution. Therefore, according to another aspect of the present invention, a method is provided for making nanoparticles of a metal halide salt doped with one or more rare earth elements by:
dissolving a water-soluble salt of a halide-forming metal in water with an excess of a water-soluble salt of the one or more rare earth element dopants, so that an aqueous solution of ions of the halide-forming metal and ions of the one or more rare earth element dopants is formed;
dissolving in the aqueous solution an excess of an ammonium halide; and
precipitating from the aqueous solution nanoparticles of a metal halide salt doped with one or more rare earth elements.
In the present invention, the active ions entirely reside in individual low-phonon energy halide nanoparticles, thereby not being influenced by the ions of other particles. Incorporation of the nanoparticles of the present invention into a passive host matrix thus obviates the problems encountered with ion-ion energy transfer, cross-relaxation, upconversion, and the like, when each of the active species reside in their respectively doped particles.
Therefore, according to yet another aspect of the present invention, a composite is provided in which the nanoparticles of the present invention are dispersed as a guest in a polymer, glass or crystalline matrix that is chemically inert thereto and optically transparent to wavelengths at which excitation, fluorescence or luminescence of the rare earth element occurs. Matrix polymers suitable for use with the present invention include thermosetting and thermoplastic organic polymers free of intrinsic optical absorptions that would be a detriment to the rare earth element absorption, fluorescence or luminescence. For example, for infrared wavelengths, non-infrared absorbing polymers may be used, such as poly(vinylfluoride) and TEFLON AF (an amorphous poly(vinylfluoride)). TEFLON PFA (a perfluoroalkoxy copolymer) may also be used. Each nanoparticle dispersed in the polymer matrix may be doped with a different active species. The composites of the present invention are easily formed and readily fiberizable.
The highly doped materials of the present invention exhibit broader absorption and luminescence than observed from corresponding prior art materials doped in lower concentrations, thereby increasing the transfer and reception of infrared signals. This broadened emission band is advantageous for many luminescent devices, which also take advantage of the versatility of a reduced phonon energy environment. The emission band can be broadened further by employing a plurality of rare earth element compositions that upon excitation, fluorescence or luminescence emit a plurality of overlapping emission bands. The emission band can also be separated into distinct spectral lines through the use of a plurality of rare earth element compositions that upon excitation, fluorescence or luminescence emit a plurality of separate and distinct emission bands.
Therefore, according to still another aspect of the present invention, a luminescent device is provided incorporating the composite of the present invention. Examples of luminescent devices include zero-loss links, wavelength-division-multiplexing devices, upconversion light sources, standard light sources, and the like. Volumetric displays based on the composites of the present invention exhibit greatly enhanced performance, easier fabrication and reduced weight.
Composites of nanoparticles doped with different active species exhibit ultra-broad band emissions attributable to the additive effects of the individual dopants. This broadened emissions band is advantageous for the fabrication of sources operating in wavelength-division-multiplexing schemes.